FeLi[PO4]: Dissection of a Crystal Structure - Inorganic 3D Structures - page 72

inorganic structures can be rationalized by assuming that electron transfer can also
occur between any pair of cations, even if they are of the same kind [ 4 ] .
By applying this concept to the Ni 2 In structure, the Ni(1) atom transfers two
electrons to the In atom, so that the Ni(1) 2+ becomes a pseudo-Fe atom (

C -Fe) and

the In 2 becomes -

Sb] substructure is of the NiAs type. Note that the real NiSb compound is also of
NiAs type! Consequently, Ni 2 In can be described as a Ni-stuffed NiAs structure.
The stuffing Ni(1) atoms occupy the centre of trigonal bipyramids, as seen in Fig. 4 .
When this model is applied to FeLi[PO 4 ], we see that the hcp array is now formed
by the P atoms, and the octahedra are filled by Li atoms. In this case, the Fe atoms
are at the centre of the trigonal bipyramids, acting simultaneously as donor and
stuffing agents. This is illustrated in Fig. 5 , where the distortion of the hcp array of
P atoms is made evident. The P octahedral network (Fig. 5a ) is very irregular
and flattened.

C -Sb. Thus, Ni 2 In converts into C -Fe[Ni C -Sb], where the [Ni C

b

a

Fig. 4 (a) The NiAs structure viewed along [110]. Red lines connect the As atoms ( grey circles )
which form an hcp array with all octahedra filled with the Ni atoms ( green ). (b) The Ni 2 In structure
viewed along [110]. The Ni(1) atoms ( dark yellow spheres ) are stuffing the [Ni C -Sb] subarray to
form the NiAs (NiSb) structure. The blue contacts form the Sb octahedra. The red lines mark the
trigonal prisms of Ni atoms which are occupied by the As atoms. Note that the NiAs structure is
implicit in Ni 2 In

a b

Fig. 5 (a) The FeLiP array in the ambient pressure phase of FeLiPO 4 (triphylite), showing the
existence of a latent NiAs-type structure formed by both the P ( red-violet ) and Li ( light grey )
atoms. Blue lines connect the P atoms ( hcp array) to depict the octahedral holes which are occupied
by the Li atoms. The Fe atoms are at the centre of triangular faces. Colour codes as in previous
figures. (b) The structure of LiClO 4 , showing its similarity with the isoelectronic (LiPO 4 ) 2 . Grey
spheres represent Li atoms. Green and red spheres represent the ClO 4 groups